With respect to a device, such as a precision optical system, in the case where a member composed of an organic resin material is used for an optical element or its peripheral member, a large dimensional change of the member due to a temperature change may cause the positional shift of the optical system. A reduction in the linear expansion coefficient of the organic resin material is effective in inhibiting the dimensional change of the member due to the temperature change. A desired linear expansion coefficient depends on the optical system. For example, in the case of an optical member for use in a catoptric system, the optical member can have a linear expansion coefficient of 50×10−6/degrees (Celsius) or less.
To reduce the linear expansion coefficient of the organic resin material, a method for adding fine inorganic particles having a low linear expansion coefficient to an organic resin is known. For example, PTL 1 discloses that in a method for reducing a linear expansion coefficient by adding fine inorganic particles to a thermoplastic organic resin, the addition of the fine inorganic particles to the resin results in a linear expansion coefficient of 9.2×10−6/degrees (Celsius) or less.
In the case where an organic resin material is used as an optical element itself, the optical element is further required to have an optically effective surface with satisfactory surface smoothness. The desired surface smoothness depends on the optical system. For example, in the case of an optical element for use in a catoptric system, the optical element is required to have an average root-mean-square surface roughness (Rq) of 10 nm or less.